Art Cashin says bitcoin may be approaching a 'trapdoor' that sends it even lower

David A. Grogan | CNBC

Art Cashin told CNBC on Thursday investors need to keep a close watch on the price of bitcoin, as the world's largest cryptocurrency by market value fell on the first day of the third quarter.

Bitcoin traded below $33,500 a little more than a week after it plunged below the key support level of $30,000. On June 22, bitcoin briefly went negative for the year — sinking below $29,000 — before rallying later in the day and ultimately turning positive.

"It looks like it may be getting ready for a retest of $30,000, and that could be critical," Cashin said on "Squawk on the Street."